As states debate whether to expand their Medicaid coverage under President Obama’s Affordable Care Act, a new study by Harvard School of Public Health researchers finds that doing so may save thousands of lives.

The study compared health statistics in three states that recently expanded their Medicaid programs — Arizona, Maine and New York — with neighboring states that did not and found that when health insurance was extended to more low-income residents, death rates dropped by about 6%.

Currently, Medicaid, a federal-state program for poor or severely disabled people, covers about 60 million Americans. Under the Affordable Care Act, Medicaid eligibility will extend to millions more Americans — perhaps half of the 30 million who are now uninsured — starting in 2014. Last month, the Supreme Court ruling on the health care law gave states the option to accept or reject the law’s Medicaid expansion, intended to grant coverage to uninsured adults without children and with annual incomes of about $15,400. Most states are expected to extend their coverage to some degree.

The debate over Medicaid expansion has largely centered around cost and states’ rights. For the first three years of expansion under the new health care law, the federal government will pick up the tab, then scale back to give states more of the financial responsibility. Some critics argue that the costs aren’t justified because coverage under the program doesn’t lead to better health; they contend that because Medicaid pays doctors less than Medicare or private insurance, patients may not get the care they need.

Enter the new study, published in the New England Journal of Medicine, which seeks to answer that very question. The study looked at three states that had expanded their Medicaid programs between 2000 and 2005 to cover low-income adults without kids or disabilities, a population that was previously ineligible for Medicaid. The researchers compared mortality rates in these three states five years before and after the expansions. They then compared the rates to four neighboring states that did not expand coverage: Pennsylvania, Nevada, New Mexico and New Hampshire.

In the five years before expansion, there were about 46,400 deaths per year in 20-to-64-year-olds in the three states combined; in the five years after, there were 44,900. For every 176 additional adults covered by Medicaid, one death per year was prevented, the study found. In the other four states where coverage was not expanded, death rates went up.

Drops in death rates were greatest among adults aged 35 to 64 years, minorities and residents of poor counties, populations who would be most affected by the expansion. The study also found that delays in care were 21% less likely to be blamed on cost, after the expansion. “Policymakers should be aware that major changes in Medicaid — either expansions or reductions in coverage — may have significant effects on the health of vulnerable populations,” the researchers write.

Because the study did not randomly assign its participants, the researchers can’t conclude that the Medicaid expansion caused the decline in mortality rates. The authors write:

One possible explanation for these findings is that expanding coverage had positive spillover effects through increased funding to providers, particularly safety-net hospitals and clinics. Publicity about the expansion may also have encouraged uninsured higher-income and elderly persons to obtain insurance from other sources, including those over the age of 65 years who did not meet lifetime earnings requirements for Medicare.

But the findings do fall in line with those of another study in Oregon, which also expanded Medicaid coverage in 2008. In that state, recipients were randomly chosen via lottery; compared with those who did not get coverage, they were more likely to receive regular medical care, including preventive screenings, and report better health.

It should be noted that the new study’s lead author, Dr. Benjamin Sommers, is currently working as an adviser in the Obama administration’s Department of Health and Human Services, but the research was completed before he left Harvard for the HHS stint.